1. Field of the Invention
The present invention relates to an Nb3Sn-based superconductive wire, and particularly, to an Nb3Sn-based superconductive wire which, when used in a superconductive magnet, manifests sufficient strength also against force along the radius direction in operating the magnet and reveals little deterioration in properties due to mechanical strain ascribable to the force along the radius direction.
2. Prior Art
Nb3Sn-based superconductive wires have a critical magnetic property of about 25 T, and widely used in superconductive magnets of 10 T or more. However, superconductive wires composed of compounds such as Nb3Sn and the like have a defect that properties, particularly, critical current property deteriorates by mechanical strain. In a superconductive magnet, electromagnetic force acts constantly on a superconductive wire in the magnet due to the magnetic field of the magnet and energizing current.
Further, since superconductive magnets made of Nb3Sn-based superconductive wires are finally formed by thermal treatment for producing an Nb3Sn-based compound, at 600 to 750° C. for a long period of time, sufficient strength is not obtained by general Nb3Sn-based superconductive wires.
Consequently, in designing of superconductive magnets made of Nb3Sn-based superconductive wires, complicated designing in view of change of properties of the wire due to the action of electromagnetic force is required. Therefore, development of an Nb3Sn-based superconductive wire having high strength property is desired.
FIG. 1 is a sectional view schematically showing the constitution of a conventional Nb3Sn-based superconductive wire.
As shown in FIG. 1, as a method of increasing the strength of an Nb3Sn-based superconductive wire, there is suggested a superconductive wire in which Ta is placed as a center reinforcing material 4 at the center of the section of a so-called bronze method Nb3Sn-based superconductive wire having an aggregate 3 formed by placing a lot of Nb or Nb alloy filaments in a copper-tin-based alloy matrix and having stabilizing copper 1 and diffusion barrier 2 on the periphery thereof (T. Hase et at., Bronze Route Conductors for 1 GHz NMR Superconducting Magnet IEEE Trans. Appl. Supercond., Vol. 10, No. 1, pp 965-970 (2000)). The aggregate 3 is produced by inserting a lot of Nb or Nb alloy filaments into a copper-tin-based alloy tube and performing diameter-reduction process on this.
Generally, in a superconductive magnet, mechanical forces act on a superconductive wire in winding by electromagnetic force along the axis direction and radius direction, and of them, regarding strain by force along the axis direction, the above-mentioned suggestion in which strength along the axis direction of a superconductive wire is increases is effective.
However, the wire according to the above-mentioned suggestion has a problem that there is a tendency of deterioration in properties of a superconductive wire, particularly, critical current property, due to strain by force along the radius direction since strength along the radius direction is equivalent to that of a wire containing no reinforcing material.